how to work it out in Gimp?
If turning the layer opacity down doesn't fix it, make sure you have only the top layer (in Gimp) set to subtract. The bottom should be normal.

However, I don't think that's your problem. I think you are just using the musgrave and wave from Gimp, which has a different seed value (and maybe a different algorithm). Plug the texture directly into an emission shader set to strength of 1, and render that. Then do so for the other texture. Be sure to save both rendered images!
Now load them into Gimp as layers.

I hope this helps. Because I don't have the files, I can't think of anything else.